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The isonymy structure of two Venezuelan states, Sucre and Tachira, is described using the surnames of the Register of Electors updated in 1991. The frequency distribution of surnames pooled together by sex was obtained for the 57 counties of Sucre and the 52 counties of Tachira, based on total population sizes of 158,705 and 160,690 individuals respectively. The coefficient of consanguinity resulting from random isonymy ((/>„), Karlin and McGregor’s /?, (= v), and the proportion of the population included in surnames represented only once (estimator A) and in the seven most frequent surnames (estimator B) were calculated for each county. RST, a measure of microdifferentiation, was estimated for each state. The Euclidean distance between pairs of counties within states was calculated together with the corresponding geographic distances. The correlations between their logarithmic transformations were significant in both cases, indicating differentiation of surnames by distance. Dendrograms based on the Euclidean distance matrix were constructed. From them a first approximation of the effect of internal migration within states was obtained. Ninety-six percent of the coefficient of consanguinity resulting from random isonymy is determined by the proportion of the population included in the seven most frequent surnames, whereas between 72% and 88% of Karlin and McGregor’s nt for Sucre and Tachira, respectively, is determined by the proportion of population included in surnames represented only once. Surnames with generalized and with focal distribution were identified for both states, to be used as possible indicators of the geographic origin of their carriers. Our results indicate that Tachira’s counties, on average, tend to be more isolated than Sucre’s counties, as measured by RsT, estimator B, and u. Comparisons with the results obtained for other Venezuelan states and other non-Venezuelan populations are also given.